TiO₂ hollow spheres on reduced graphene oxide with high rate performance as anodes for lithium-ion batteries

Abstract

Anatase TiO₂ anchored on graphene oxide (GO) can be synthesized through a one-step hydrothermal method. The as-formed nanohybrid has a unique hollow structure and a large surface area. More importantly, compared to the pristine TiO₂ counterpart, TiO₂@RGO composite materials as anodes in lithium-ion batteries have demonstrated a uniform and highly crystallized morphology and exhibited excellent cycling stability and rate capability of 352 mA h g⁻¹ at 0.5C and 223 mA h g⁻¹ at 5C after 100 cycles, indicating that the TiO₂@RGO nanocomposite has promise in advanced Li-ion batteries. The improvement of electrochemical performance is assigned to the enhanced conductivity in the presence of GO in the TiO₂@RGO nanocomposite, the anatase and TiO₂–B mixed crystal phase of the hollow sphere TiO₂@RGO nanocomposite, the small size of TiO₂ particles in the nanocomposite, and the enlarged electrode/electrolyte contact area, leading to more active sites in TiO₂@RGO.

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Introduction

Lithium ion batteries (LIBs) have gained growing interest because of their wide applications in portable electronic devices as well as electronic vehicles due to their high capacity, long cycle life and environmental benignity.^1–3 Particularly, titanium dioxide (TiO₂), has been attracting much attention because of its low price, non-toxicity and low volume expansion (<4%) during the Li-ion insertion–extraction processes.^4–6 In addition, owing to the higher Li-ion intercalation potential (1.5–1.8 V versus Li⁺/Li) of TiO₂, the generation of lithium dendrites could be suppressed. Its reaction process can be described properly with the following equation: TiO₂ + xLi⁺ + xe⁻ → 2Li_xTiO₂ (0 ≤ x ≤ 0.5). However, the theoretical capacity of anatase and rutile phase TiO₂ is only 168 mA h g⁻¹.^7–9 Meanwhile, the reunion and decomposition of TiO₂ during charge–discharge cycles can result in the reduction of active sites and the degradation of specific capacity, which definitely hinder the application of TiO₂ in LIB anode materials.¹⁰ In order to improve electrical and ionic conductivities, adding some conductive materials, for example, carbon nanomaterials, metals, metal oxides and polymers, is a common practice. In this way, we can shorten the transmission distance and decrease the resistance of intrinsic electrons and lithium ions. At same time, different structures of TiO₂, such as 1D nanowires or nanotubes, 2D nanosheets and even complicated 3D structures, can get higher capacities and high rate capabilities in the electrodes of lithium ions batteries based on following reason: higher surface area which is beneficial to increase the contact area between electrolyte and electrode, resulting in an excellent cycling performance of electrode materials.

Compared with conventional conductive additives, graphene, with a ultrathin two-dimensional carbon sheet, has lots of unique properties, for example, high electronic conductivity, large specific surface area and high structural flexibility.^11–13 Due to the TiO₂ loading on the graphene showed excellent electrochemical performance such as high rate capacity, shortened transport length of electrons, and enhanced contact area of electrolyte–electrode, TiO₂/GO nanocomposites have been achieving a wide attention for exploring Li storage based on above advantages.^14–16 Li et al. adopted the hydrothermal method to fabricate mesoporous anatase TiO₂ nanospheres anchoring onto graphene sheets, which demonstrated a reversible capacity of 200 mA h g⁻¹ at the rate of 1C after 100 cycles.¹⁷ Zhen et al. used hydrothermal process to construct mesoporous structure TiO₂/RGO hybrid materials, which possessed an excellent capacity retention about 260 mA h g⁻¹ at 1.2C after 400 cycles.¹⁸ However, most of their experimental process are quiet complex and are not easily operated in the practical application. Therefore, exploring a simple, facile and easy-to-handle method to synthesize well-dispersed TiO₂/GO hybrid anode materials which possessed properties of structural stability and great performance is particularly important.

In this paper, we propose a simple strategy to fabricate nanostructured hollow spheres TiO₂@RGO anode materials which make nanocrystalline TiO₂ grown in situ on graphene via a facile template-free one-step followed by calcinations.

Compared with the reported hydrothermal–solvothermal routes, the current experiment schemes were performed without addition of any surfactants. It only use tetrabutyl titanate(Ti(OR)₄), LiOH and GO as the raw materials. What is novel about our work is that there are few literatures on the synthesizing nanostructured hollow spheres TiO₂@RGO electrode used TBT as precursor via template-free one-step in situ method which was deployed in LIBs. It delivers the characteristics of hollow structures with large surface area and highly mesoporous structures. We significantly improve the electrochemical performance of anode materials and preparation procedures of TiO₂@RGO composite materials. As a result, the excellent electrochemical performance of as-prepared TiO₂@RGO electrode materials shows its great potential values in advanced Li-ion batteries under a high-rate current density. Scheme 1 illustrates the synthesis process of the TiO₂@RGO nanocomposite.

Scheme 1 Scheme of the synthesis TiO₂@RGO nanocomposite.

Experiment

Preparation of GO

Graphene oxide was prepared via the oxidation of graphene by applying improved Hummers' method.⁴⁴

Preparation of nanostructure TiO₂@RGOcomposites

In a typical synthetic procedure, 50 mg GO was dissolved in 30 mL solution containing 5.0 g LiOH, followed by ultrasound for 2 h. Then, 2 mL tetrabutyl titanate (TBT) was dispersed into the above solution drop by drop along with vigorous agitation for 30 min and ultrasounded for another two hours. Thereafter, the mixture was placed into a 50 mL Teflon-lined autoclave and maintained at 130 °C for 48 h. After naturally cooled to room temperature, the obtained products were soaked in 0.1 M H₂SO₄ solution for some time, centrifuged and washed several times by distilled water and ethanol until the pH value was close to 7, then dried at 60 °C in oven for 24 h and annealed at 450 °C for 4 h under nitrogen to get the final product, namely, the TiO₂-reduced graphene oxide (termed as TiO₂@RGO) nanocomposite. The fabrication of the pure TiO₂ nanoparticles, a similar process was proceeded without the addition of GO in the original solution.

Material characterizations

The X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advance X-ray Diffractometer. Raman spectroscopy (Renishaw, 633 nm excitation) was used to characterize the interaction among of TiO₂ and GO. Structure and morphology of TiO₂@RGO-Nd nanohybrid, the TiO₂@RGO and the pure TiO₂ were observed by a transmission electron microscope (TEM; JSM-2100F) and a Cold Field Scanning electron microscope (SEM; JSM-7500F). Nitrogen adsorption/desorption tests were conducted at Brunauer–Emmett–Teller (BET) measurements (JW-BK222 surface area analyzer).

Electrochemical measurements

The electrochemical measurements were carried out with two-electrode CR2025 coin cells. The working electrode was prepared by mixing 80 wt% active material (TiO₂@RGO), 10 wt% carbon black and 10 wt% polyvinylidene fluoride (PVDF). Then the mixture was dissolved in N-methyl-2-pyrrolidone under stirring. The slurry was coated onto a Cu foil and dried at 120 °C for 12 h. The loading mass of active material was approximately 0.43 mg cm⁻². The electrochemical measurements were carried out with two-electrode CR2025 coin cells. The lithium foil was used as the counter and reference electrodes, Celgard 2300 as the separator and 1 M LiPF₆ as the electrolyte. Constant current charge–discharge tests were carried out on a Land Battery Measurement System (CT2001A) at various current densities with a cut off voltage of 1–2.5 V at room temperature. Cyclic voltammograms (CV) at a scan rate of 0.2 mV s⁻¹ and electrochemical impedance spectra over a frequency range of 100 kHz to 0.1 Hz were measured on a CHI660E electrochemical work station at room temperature.

Results and discussion

Morphology and structural characterization

The crystallographic structure of the samples can be provided by X-ray diffraction (XRD) patterns. As shown in Fig. 1a, the pure TiO₂ exhibits good crystallinity. The peaks at 25.3°, 37.8°, 48.1°, 55.1° and 62.8° correspond to the lattice planes of (101), (004), (200), (211), (204), respectively,¹⁹ indicating that the TiO₂ is anatase phase (JCPDS no. 21-1272).^20,21 At the same time, the XRD patterns of the TiO₂@RGO nanocomposite retain the same position of the diffraction peaks.

Fig. 1 (a) XRD patterns of TiO₂@RGO, TiO₂ and (b) Raman spectra of TiO₂@RGO and RGO.

However, the diffraction peaks of the TiO₂ in the TiO₂@RGO nanocomposite become broader and lower, demonstrating a small crystallite size. Meanwhile, the XRD spectrums of hybrid nanocomposite and pure TiO₂ all display a typical TiO₂–B diffraction peaks (space group C2/m, JCPD no. 35-0088) at 14.3°, 25.0°, 28.7°, 44.7° and 58.4° corresponding to the (001), (110), (002), (−511), (−421) reflections, respectively.²² The skeleton of the TiO₂–B structure is consisted of corrugated sheets of edge and corner-sharing TiO₆ octahedra that are linked by bridging oxygen atoms, formed a three-dimensional network. This nanostructure is more open than those of rutile and anatase, which makes the material an effective host for lithium ions storage.^23,24 One diffraction peak centered at 2θ = 11.41° corresponds to the (001) reflection of GO with an interlayer spacing of 7.75 Å.²⁵ The peak disappeared in the following compound process, indicating that the GO nanosheets have been completely reduced to RGO nanosheets using the current hydrothermal treatment. In the same position, the TiO₂@RGO nanocomposite don't have the characteristic peaks of GO, showing that the TiO₂ uniformly dispersed on the GO nanosheets. Fig. 1b shows the Raman spectra of GO before and after deposition of TiO₂. The GO exhibited Raman shifts at approximately 1354 and 1570 cm⁻¹ corresponding to the D and G bands, respectively. After heat-treatment at 450 °C, the D and G bands of TiO₂@RGO located at about 1358 and 1594 cm⁻¹. The intensity of D band to G band (I_D/I_G) for the TiO₂@RGO (1.06) is a bit higher than that of GO (0.85). The intensity of D band to G band (I_D/I_G) for the TiO₂@RGO (1.06) is a bit higher than that of GO (0.85), indicating the reduction of GO in the TiO₂@RGO nanocomposite.

When TBT was added into GO sheets, the TiO₂ nanoparticles homogeneously anchored on RGO nanosheets. The morphology of TiO₂@RGO hybrid electrode material, GO and pure TiO₂ were observed via scanning electron microscopy (SEM). It can be clearly seen that GO has a wrinkled flake-like structure in Fig. 2a and b, which is in accorded with the results in the recent article.^45,46 As shown in the Fig. 3a and c, the TiO₂ were regularly spherical. Fig. 3c shows that the TiO₂ nanospheres are well-dispersed and uniformly attached on the surface of the RGO nanosheets. The TiO₂ exhibited good dispersibility in the pure phase and composite electrode materials. To further explore the microstructure of the materials, the products were investigated by transmission electron microscopy (TEM). It can be seen that the TiO₂ exhibits a hollow sphere structure with uniform particle size in Fig. 3b. The hollow structure of the TiO₂ nanocomposite remained unchanged as shown in Fig. 3d. There is no structure collapse before and after the composite. The hollow structure can be seen clearly in Fig. 3b and d. The wall thickness of TiO₂ hollow nanostructures is about 2–4 nm. The TiO₂ possesses nearly spherical structure with the size of particle about 10 nm and deliver good features, such as more stable, active site distribution more uniform, high specific surface area, wall thickness in 2–4 nm, which are contribute to the ions and electrons transfer, beneficial to the improvement of the capacity. The combination of TiO₂ materials with such structure advantages and graphene materials will greatly increase the performance of TiO₂ as anode material. The selected area electronic diffraction nanostructure pattern (SAED) of the hollow spheres TiO₂@RGO nanostructure delivers a set of diffraction rings of the anatase and TiO₂–B structure of the TiO₂, indicating the polycrystalline nature of the products, which can be clearly assigned to the anatase TiO₂ and TiO₂–B, respectively. This result is consistent with the XRD results. As shown in Fig. 3d, the TiO₂@RGO composites, in which the hollow spheres were uniformly and well dispersedly attached on the surface of RGO sheets. Fig. 3e is the High-Resolution TEM(HRTEM) image of TiO₂@RGO. As shown in Fig. 3e, the lattice fringes with the spaces of 0.35 nm and 0.62 nm correspond to the (101) and (001) of anatase TiO₂ and TiO₂–B, respectively. Elemental mapping and EDX spectrum for TiO₂@RGO nanocomposite are also shown in Fig. S1 (in the ESI†). The corresponding element mapping images (Fig. S1†) of C, Ti, and O in the TiO₂@RGO nanocomposite overlapped with each other, indicating the uniform distribution of these elements and the nanohollow sphere TiO₂ are evenly distributed across the surfaces of rGO. The corresponding EDX of the selected area is also depicted in Fig. S1e,† which agrees well with the mapping results, except for the Si peaks that originated from silicon substrate. Based on the EDS elements mapping analysis, we confirm that it has a unique hollow structure.

Fig. 2 SEM and TEM images of bare GO.

Fig. 3 SEM (a) and TEM (b) images of pure TiO₂, SEM (c) and TEM (d) images of TiO₂@RGO nanocomposite, HRTEM (e) and SAED (f) images of TiO₂@RGO.

Nitrogen isotherm adsorption–desorption curves with the pore size distributions of the pristine TiO₂ and TiO₂@RGO are presented in Fig. 4a and b. For TiO₂@RGO, a type IV isotherm is observed, which is characteristic of mesoporous materials.⁴⁹ Based on the Barrett–Joyner–Halenda (BJH) equation, the main pore size (inset in Fig. 4b) in TiO₂@RGO is 8.5 nm less than 11.6 nm of the pure TiO₂(inset in Fig. 4a). It is in agreement with the pore size determined from the TEM images and the results described in the XRD, which further confirms a uniform pore size distribution. Compared to the specific surface area (105.5 m² g⁻¹) of the TiO₂, the specific surface area of TiO₂@RGO is 145.8 m² g⁻¹. The larger surface area of TiO₂@RGO is conductive to increasing contact area of electrode–electrolyte and can improve diffusion of electrolyte ions.

Fig. 4 Nitrogen isotherm adsorption–desorption curves of (a) the pure TiO₂ and (b) TiO₂@RGO. The inset in (a) and (b) show pore-size distributions of the pure TiO₂ and TiO₂@RGO.

The electrochemical performance of the prepared pure TiO₂ and TiO₂@RGO materials were investigated as electrode for LIBs. Fig. 5a and b shows the discharge–charge curves of the pure TiO₂ and TiO₂@RGO at a rate of 0.5C (1C = 168 mA h g⁻¹) in the potential window of 1.0–2.5 V versus Li⁺/Li in the 1st, 2nd, 10th, 50th and 100th. The initial discharge–charge capacities of the pure TiO₂ were 570/399 mA h g⁻¹, and subsequently deceased to just 223 mA h g⁻¹ after 100 cycles. As a comparison, the TiO₂@RGO hybrid nanocomposite obtained higher capacities with 634/388 mA h g⁻¹ at the first cycle. The columbic efficiency of the pure TiO₂ and TiO₂@RGO is comparatively low in the first cycle, which can be attributed to the existence of irreversible Li trapped sites.²⁶ There are two obvious plateaus at 1.75 V and 1.95 V, corresponding to Li⁺ insertion and Li⁺ extraction between the TiO₂ nanoparticles and Li_0.5TiO₂, which meet the characteristics of anatase TiO₂ lithiation.^47,48 Meanwhile, it is obvious to see that there are huge differences of plateau voltage between pure TiO₂ and TiO₂@RGO hybrid electrode, indicating that the TiO₂@RGO nanocomposites possessed excellent reaction kinetics and low polarization. Compared with TiO₂@RGO electrode, the redox potential plateaus of pure TiO₂ is getting smaller and smaller until it disappeared in the end because of its bigger size and inherently bad electronic conductivity. Fig. 5c and d depicts the cycling performance of the pristine TiO₂ and TiO₂@RGO electrode at a rate of 0.5C and 5C. Just as showed in Fig. 5d, the TiO₂@RGO hybrid composite displays excellent reversible capacity and cycle characteristics. The results demonstrated that the TiO₂@RGO electrode obtained high reversible capacity of 352 mA h g⁻¹ (0.5C) and 223 mA h g⁻¹ (5C) up to 100 cycles, which were higher than that of anode materials composed of anatase TiO₂ nanospheres/hollow structures, rutile TiO₂ nanorods/porous microspheres/nanoparticles, etc.^27–32 At the same time, the initial discharge–charge capacities of pure anatase TiO₂ were 570/399 mA h g⁻¹, which demonstrate that the pure phase TiO₂ has high specific capacity. The first discharge capacity of TiO₂ is about 570 mA h g⁻¹, a value much higher than the theoretical capacity of TiO₂. It can be explained that the specific surface area of pure TiO₂ is 105.5 m² g⁻¹ and the specific surface area of TiO₂@rGO is more than 145.8 m² g⁻¹. The large specific surface area plays an important role in the consumption of lithium ion at initial discharge process. At the same time, the formation of the SEI film will consume a large amount of lithium ion. The reasons for the above statement will lead to the increase of capacity. Subsequently, discharge capacities of pure anatase TiO₂ quickly decreased to 129 mA h g⁻¹ at 5C and about 223 mA h g⁻¹ at 0.5C (after 100 cycles) accompanied by capacities rose and downed which could be attributed to the activation and the structure collapsed of active materials.³³ Meanwhile, on the other hand, we can see that the pure TiO₂ electrode has the potential value based on its high initial capacities. Compared with pure TiO₂ electrode, TiO₂@RGO shown a higher rate performance and ultra long cycle life for lithium storage, which could be attributed to its unique hollow nanostructure, relatively large surface areas (the particle size of sample is about 8.5 nm). Also, RGO nanosheets can benefit to restraining grain growth and inhabit anatase TiO₂ aggregation. Fig. 6 shows the rate performance of the pure TiO₂ and TiO₂@RGO hybrid composites at different rates. When cycled at the rate of 0.5, 1, 2 and 5C, the reversible discharge capacities of the pure TiO₂ were 263, 229, 198, and 147 mA h g⁻¹, respectively. Compared with pure TiO₂ electrode, the TiO₂@RGO hybrid composites owned higher reversible capacities of 585, 509, 434, 354 mA h g⁻¹, respectively. Especially, there are no obvious capacity recession when the rate returned to 2, 1 and 0.5C, which demonstrated that TiO₂@RGO hybrid composites possess excellent electrochemical properties even at high rates due to the superiority of taken experimental methods and unique features of hybrid nanostructure. The performance of TiO₂@RGO hybrid composites were superior to other the report results.^34,35 As a typical example, Lee et al. fabricated TiO₂ nanorod arrays on RGO as LIB anodes, and the capacity was only 94 mA h g⁻¹ at 5C less than the capacity of 354 mA h g⁻¹ at 5C in this article.³⁶ In order to verify the characteristics of the synthesized electrode materials, the electrochemical performance of the nanohybrid was analyzed by the charge/discharge cycle at a high rate. As shown in Fig. 6b, the charge–discharge curve was well fitted and the capacity remained at 217 mA h g⁻¹ at a rate of 10C after 2000 cycles, indicating a good capacity retention, high specific capacity, good cycle reversibility and stability of the material.

Fig. 5 Discharge–charge profiles of (a) pure TiO₂ and (b) TiO₂@RGO; cycling performance of (c) pure TiO₂ and (d) TiO₂@RGO at 0.5C and 5C.

Fig. 6 (a) Rate performance of TiO₂@RGO nanocomposite and pure TiO₂ at various current densities between 0.5C and 5C. (b) Durability test at high 5C and 10C currents of the TiO₂@RGO at 5C and 10C.

In order to further explore the synergistic effects of RGO nanosheets and the pristine TiO₂, the CV curve was tested for the first three cycles of the Li-ion half-cell at a voltage range of 1–2.5 V vs. Li/Li⁺ at a scanning speed of 0.1 mV s⁻¹. As shown in the figure, hybrid TiO₂@RGO spectrums have three pairs of obvious oxidation (insertion) and deoxidization (extraction) peaks, with the first red-ox peaks of 1.49/1.54, 1.57/1.68 and 1.73/1.98 V, respectively. The second and third cycles of red-ox peaks correspond to 1.49/1.54, 1.57/1.68, 1.73/1.98 V. The red-ox peaks appearing on the first cycle are shifted relative to the second and third cycle, which is due to the initial decomposition of the electrolyte in the hybrid electrode and the formation of the mesophase of the solid electrolyte interphase (SEI).^37,38 The pair of cathodic/anodic peaks at 1.73/1.98 V correspond to the anatase TiO₂ electrode. The relatively weak peaks at 1.49/1.54 and 1.57/1.68 V correspond to the red-ox peaks for Li/Li⁺ embedded/out in the TiO₂–B, indicating the content of Li/Li⁺ is relatively less than that of anatase TiO₂. It agrees with the results of XRD analysis. The CV curve of the first cycle and the next two cycles CV spectrums suggest the first irreversible charge–discharge capacity of hybrid electrode and lost capacity (Fig. 7). The second and third CV curves are well overlapped, which indicate that the hybrid electrode has excellent cycle performance, high reversibility and good capacity retention.

To gain insight into the remarkable rate performance of the TiO₂@RGO composites, electrochemical impedance spectroscopy (EIS) was performed on the cells consisting of pure TiO₂ and TiO₂@RGO composites as the working electrode vs. Li without discharge and charge cycles before cycles. Fig. 8, the Nyquist plots display an inclined line in the low-frequency range and only one semicircle in the high-frequency range. The inclining line and high frequency oblate semicircle correspond to the Li⁺ diffusion process and charge-transfer impedance in the electrode/electrolyte interface, respectively.³⁹ Compared with pure TiO₂ (132 Ω), TiO₂@RGO nanostructure has a smaller resistance value of 69.2 Ω that can effectively improve the electron transport.

Fig. 7 CV curve of (a) pure TiO₂ and (b) TiO₂@RGO nanocomposite.

Fig. 8 Nyquist plots for the EIS of TiO₂@RGO and pure TiO₂ electrodes before cycling.

The enhanced lithium storage performance could be assigned to the following reasons. On the one hand, the fabrication of the anatase TiO₂ itself has a unique hollow crystal nanostructure that facilitate the Li ion diffusion owing to the open channels and the shortened transport pathways, a small grain size (less than 10 nm) and a large surface that which benefits to the contact area between the electrode and the electrolyte. On the other hand, the functional active groups on the surface of RGO nanosheets, such as –OH, –COOH, are conductive to adhesion of the hollow anatase TiO₂ which can reduce the aggregation of TiO₂ and make the TiO₂ obtain a good dispersibility. At the same time, it benefits the combination of RGO nanosheets and the hollow anatase TiO₂ that can achieve a more stable hybrid nanostructure composite. The aggregation of TiO₂ and stacking of GO can be effectively prevented, leading to good cycling performance. Last but not least, the conductivity of RGO nanosheets with large surface and the layered structure is an effective tool for solving the poor electrical conductivity of pure TiO₂, enhancing the quality of the electrode–electrolyte contact, providing abundant nanoporous channels for Li⁺/electron transfer and preventing the aggregation of anatase TiO₂.^40–43 The synergistic effect of nanostructured TiO₂ and RGO is a main factor for the superior rate capability and excellent cycle ability of the TiO₂@RGO nanohybrid.

Conclusions

We have developed a facile in situ hydrothermal synthesis approach to construct a hollow TiO₂@RGO nanohybrid composite. The combination of the TiO₂ has an open crystal structure, a small particle size and a large surface. Compared with the pristine TiO₂ and other TiO₂@RGO nanocomposites prepared through hydrothermal/solvothermal approaches, the as-prepared TiO₂@RGO nanocomposite shown significantly enhanced lithium storage performance, much higher reversible capacity, and better cycling stability. More importantly, the as-prepared TiO₂@RGO nanocomposite still showed a higher reversible capacity for 220 mA h g⁻¹ at 5C current after 100 cycles. The simple and steerable synthesis, excellent electrochemical performance, and the hollow sphere structure could also provide a new idea for developing higher-performance lithium storage electrodes and other energy storage applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Science Foundation of China (51275203).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra10681e

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